1. Field of the Invention
The invention relates to a valve secured to a rotatable shaft that enters a liquid-containing vessel, such as a mixing vessel. More specifically, the present invention relates to a valve that temporarily seals the vessel to facilitate servicing or replacement of the mechanical seal, which is the primary seal between the vessel and the shaft.
2. Description of the Prior Art
The invention relates to a shut-off valve/sealing device for a rotatable shaft that enters a vessel which contains a fluid therein, such as a liquid or a gas, or any other type of flowable material, the fluid being mixed through an opening in the vessel below the level of the fluid. The invention is particularly applicable to large fluid-containing mixing vessels which accommodate an agitator or impeller driven by a motorized shaft. The drive means for driving the rotatable shaft is located outside the vessel with the drive shaft extending through an opening in the vessel, the opening generally being below the level of the fluid contained inside the vessel. The drive means is normally at a relatively low level on the vessel (in other words towards the base of the vessel), thereby necessitating the use of a seal, such as a mechanical seal or stuffing box, around the shaft at the shaft's entry point via the opening into the vessel in order to prevent fluid leakage or escape. Such rotating shafts are often times equipped with a sealant packing contained within a stuffing box housing about the shaft at the shaft's point of entry into the vessel. When the packing is no longer functioning properly, it must be replaced. The shaft may also be equipped with a rotary seal which is subject to normal wear and tear from use near the point of entry of the shaft into the vessel. The rotary seal includes various components which inevitably need replacement or repair.
Typically, when the worn packing or components of the rotary seal or stuffing box need replacement, the entire contents of the mixing vessel must be drained or otherwise removed since the removal of the packing breaks the seal about the shaft at its point of entry into the vessel. The emptying of the vessel is a time-consuming and expensive process and also can potentially cause unnecessary and undesirable waste of some or all of the contents of the vessel, contamination of the vessel contents, or exposure to potentially harmful liquids or noxious gases to persons servicing the seal or tending to the vessel.
U.S. Pat. Nos. 4,878,677 and 5,303,933 (Larkins et al.) and (Larkins), describe similar devices for providing a shut-off seal for a rotatable shaft without disturbing the housing or rotary seal, as the case may be. Jacking screws are employed, which reach through the flange, for engagement with a shut-off ring. Such devices require the draw of multiple screw jacks to be even or balanced. This is disadvantageous in that a complete and balanced seal is not ensured, thereby allowing the potential for some fluid leakage.
Another significant disadvantage with the aforementioned devices is that both devices employ O-ring seals which serve to prevent leakage around the respective device, even when the device is not in use. When put into use, the O-ring seals are dragged across surfaces that have been exposed to the vessel contents, thereby risking damage to the seals. Furthermore, such damage to the seals risks causing subsequent leakage when the device is returned to its storage position.
There is a need for an improved shut-off seal device for use with a rotatable shaft which extends into a mixing tank or vessel.